fix(windows): redraw Flutter only when the captured WebView frame changed

[Nathaniel-260]

Connection with issue(s)

Issues are disabled on this repository, so there is no issue to resolve. Downstream context: Y-PLONI/otzaria-word-editor#53 (typing latency investigation in the Word plugin, where the host-side measurements below were made).

What

On Windows the WebView is shown through a Flutter texture: WebView2 draws into a composition visual, Windows.Graphics.Capture captures that visual, and the captured frame is copied into a DXGI shared texture that Flutter samples.

Capturing a composition visual yields a frame on every compositor tick, whether or not the WebView repainted. TextureBridge::OnFrameArrived forwarded each of them with MarkTextureFrameAvailable, so Flutter re-rasterized at the display refresh rate for as long as any WebView was on screen, and GetSurfaceDescriptor copied the frame again on the raster thread each time.

This PR:

• compares every captured frame with the frame Flutter already has, on the GPU (one compute-shader dispatch, 4-byte readback), and only copies + notifies Flutter when a pixel differs;
• stops copying on the raster thread (GetSurfaceDescriptor just hands out the shared handle);
• serializes use of the shared immediate context with a mutex on GraphicsContext (several WebViews share it).

It falls back to the previous forward-every-frame behaviour when the compute shader cannot be built (feature level < 11_0, or d3dcompiler_47.dll missing — loaded on demand, no new link dependency). The frame pool stays dispatcher-bound: a free-threaded pool was tried and delivered no frames at all with the plugin's WRL event handlers.

Measurements

Otzaria, Debug build of the same commit, same machine, same 5-page document open in the same WebView; the only difference is this plugin. Flutter frames counted from the Dart VM timeline (Animator::DrawLastLayerTrees = texture-driven redraws); latency measured with real SendInput keystrokes into the WebView and screen polling of the caret line until its pixels change.

	before	after
Flutter redraws in 5 s, page idle, caret blinking	430	18
Flutter redraws in 5 s, page idle, caret blink paused	340	0
otzaria.exe CPU while idle (percent of one core, 10 s)	19–21%	0–4%
keystroke → pixels on screen, median (20 keys)	141 ms	153 ms
renderer paint → pixels on screen, median	135 ms	139 ms
renderer paint → pixels on screen, p90	341 ms	239 ms

Latency to the screen is unchanged within noise: the hop count (Chromium → DWM → capture → Flutter → DWM) is the same. What changes is that an idle WebView no longer keeps Flutter's raster thread and the GPU busy at 60 fps.

Verification

• flutter build windows --debug of Otzaria against this branch; the WebView renders, updates on typing and caret blink, survives tab switches (pause/resume recreates the surface), and the dev-plugin reload.
• Standalone MSVC compile of the three bridge translation units with the plugin's flags (/W4, _HAS_EXCEPTIONS=0), no warnings.

Testing and Review Notes

1. Build Otzaria (Debug) against this branch and open any plugin tab.

2. Record the Dart VM timeline for 5 s with the page idle: Animator::DrawLastLayerTrees should appear only when the page changes (caret blink, typing), not ~60 times per second.

3. Type into the page, switch tabs away and back (pause/resume), resize the window, open two plugin tabs: the WebView must render and update in all cases.

4. On a machine without D3D feature level 11_0 or without d3dcompiler_47.dll the log prints WebView frame compare unavailable and behaviour is the same as before this PR.